The Inspiron 15r covers around over a dozen models. Please specify the model number. You can get the exact model number by entering the service tag on the support site www.support.dell.com This would show if the system is compatible with windows 10 and the model number as well. Also check if you have the latest BIOS firmware on the system, which will be available in the drivers and downloads section in the support site.
It's model 5537 and the support site said it was windows 10 upgrade ready. I have not downloaded the latest BIOS firmware I will try that next and let you know